Aircraft jet engines can be damaged by Foreign Object Debris (FOD) sucked into the air intake. Large objects, for example a large bird strike, can cause significant changes that can be detected by the aircraft monitoring systems, such as shaft vibration. Smaller objects however, such as pebbles or bolts can cause damage to a blade that is barely visible to the eye but weakens the blade such that under continued use the stresses result in blade failure. A method for detecting that FOD has been ingested would assist in maintenance.
Systems have been proposed which employ radar for FOD detection. Shephard, Tait, King, ‘Foreign Object Detection Using Radar’, IEEE Aerospace Conference 2000, describes such a system where radar antennas are mounted within the air intake. As the FOD is moving it can be detected by the radar using Doppler processing. A problem for such systems is that the jet engine blades are large and very fast moving. The Jet Engine Modulation (JEM) of the radar signal reflected from the blades is spread across a wide range of Doppler frequencies. The much smaller signal return from the FOD has to be detected in the presence of this JEM signal.
Standard radar techniques can be used to mitigate the effects of JEM. The antenna can be made highly directional so that the radar energy is directed away from the engine blades. Also, radar range processing can be used to suppress ranges corresponding to the distance from the antenna to the blades. However the JEM signal is very much larger than the FOD signal and will still be received through both the antenna sidelobes and the range sidelobes and so these techniques are not sufficient to adequately suppress the JEM.